Home canning equipment in widespread use includes glass jars, the finish portions of which are generally provided with four continuous external helical threads to the inch, with no thread stop to limit rotation of the closure. A typical closure comprises a two-piece assembly which includes a lid and a threaded band. The lid is a disk, typically made of sheet metal such as tin-free steel ("T.F.S."), or electrotin plate ("E.T.P."), concentrically provided on its underside with a shallow annular trough containing a sealing gasket united with the disk. After a lid is placed with its gasket in sealing relation to the end of the jar finish, a band is threaded in place to protect the sealing relationship. When the band is threadably tightened, beyond initial contact of its annular flange with the upper surface of the radially outer peripheral region of the lid, the gasket is progressively axially compressed in thickness to improve the effectiveness of the sealing relationship.
If a gasket is insufficiently compressed, the lowered degree of vacuum which can be maintained within the jar may reduce shelf life or permit spoilage of the canned food. On the other hand, if the band is threadably tightened to excess, too much of the gasket will be squeezed out from between the disk and the end of the finish and the risk of loss of the sealing relationship will increase. Between insufficiency and excess, there is a range of seal compression for which the risk of loss of the sealing relationship is minimized. That corresponds to an ideal degree of tightness for the band.
Instructions provided to home canners are typified by the 29th edition of the Ball Blue Book, which was available from Ball Corporation in mid-1976. It instructs the home canner to turn the bands until they are "firmly tight". When home canning skills are passed along by apprenticeship, the learned canner can demonstrate, observe and make sure the novice applies an acceptable degree of tightness to the band. It is a continuing goal to provide written and pictorial instructions which may be followed with ease and confidence to produce reliably canned foods even for an otherwise unaided and inexperienced person who desires to be a home canner.
A simple solution readily presents itself. A first tick mark may be placed on the side of the jar, and a second tick mark may be provided on the band, at such relative locations that the gasket is sufficiently compressed when the band is tightened just enough to align the tick marks.
However, variations both available and foreseeable in home canning equipment render the foregoing solution impractical, except for the special case wherein the user has available a supply of jars and closures that are designed for one another and remain in supply with uniform specifications over the years. Such a special case is an essential part of the proposal put forward in the prior U.S. Pat. No. 3,216,600 of Dreps.
In general, containers for home canning are subject to meeting many industry-accepted standards which tolerate interchanging of jars, lids, bands, and other closures, among brands and years of manufacture. This is a practical necessity, as it is a common expectation in home canning that jars will be used and reused several times over a period of years, but that closures, or some constituents of closures will not be reused, or will be more frequently replaced than jars. Although lids are almost never reused, bands are frequently reused, but rarely to the extent that jars are reused.
Those standards advantageously permit adoption of different materials and thicknesses for the lid gaskets. However, as a result, the amount of turning of closure bands needed to produce an acceptable compression of lid gaskets of different thickness or composition is not a single constant value in the home canning field.